1. Field of the Invention
The present invention relates to a process for preparing alkylpolyether-substituted mercaptosilanes.
2. Description of the Background
The use of silanes as adhesion promoters is known. For instance, aminoalkyltrialkoxysilanes, methacryloyloxyalkyltrialkoxysilanes, polysulphanealkyltrialkoxysilanes and mercaptoalkyltrialkoxysilanes are used as promoters of adhesion between inorganic materials and organic polymers, as crosslinking agents and surface modifiers (E. P. Plueddemann, “Silane Coupling Agents”, 2nd Ed., Plenum Press 1982, pages 1-53).
These adhesion promoters, or coupling agents or bonding agents, form bonds both to the filler and to the elastomer, and hence bring about effective interaction between the filler surface and the elastomer.
Moreover it is known that the use of commercially customary silane adhesion promoters having three alkoxy substituents on the silicon atom leads to the release of considerable amounts of alcohol during and after attachment to the filler (DE 22 55 577). Since it is usual to use trimethoxy-substituted and triethoxy-substituted silanes, the corresponding alcohols, methanol and ethanol, are released in considerable amounts (Berkemeier, D.; Hader, W.; Rinker, M.; Heiss, G., Mixing of silica compounds from the viewpoint of a manufacturer of internal mixers, Gummi, Fasern, Kunststoffe (2001), Volume 54(1), pages 17-22).
It is known, furthermore, that methoxy-substituted and ethoxy-substituted silanes are more reactive than the corresponding long-chain alkoxy-substituted silanes and are therefore able to attach more quickly to the filler, so that to date it has not been possible from a technical and economic viewpoint to do without the use of methoxy and ethoxy substituents (H. D. Luginsland, “A review on the chemistry and the reinforcement of the silica-silane filler system for rubber applications”, Shaker Verlag Aachen 2002, page 25ff.).
Furthermore, DE 10137809, JP 62-181346, DE 3426987 and EP 0085831 disclose various mercaptosilanes. DE 102005032658.7 and EP 1285926 disclose alkyl-polyether-substituted mercaptosilanes which release less methanol and/or ethanol but have the same reactivity. These silanes can be synthesized by subjecting alkoxysilanes to catalysed reaction with an alkylpolyether alcohol.
A disadvantage of the known processes for the synthesis of alkylpolyether-substituted mercaptosilanes is the pale brown to orange-red colour which occurs, which may fluctuate depending on the quality and the preparation process of the mercaptosilane used.
U.S. Pat. No. 5,210,250 discloses a method of decolouring halogenated silanes by treatment with insoluble, cationic, surface-active substances.
A disadvantage of that method is that the decolouring takes place in a step downstream from the synthesis, which in operational practice entails a not inconsiderable level of extra cost and inconvenience.
Furthermore, EP 1188758 and EP 1153927 disclose methods of influencing the colour of bis(3-[triethoxysilyl]propyl)polysulphanes and -tetrasulphanes, respectively, by adding chloropropyltrichlorosilane during the sulphurization reaction. From US 20040092758 it is known that polysulphide silanes with a low level of coloration can be obtained by increasing the reaction temperature during the sulphurization reaction. Both methods intervene in the course of the reaction at a point at which polysulphide species of different chain lengths are present or are forming in the reaction mixture. In the customary processes of mercaptosilane preparation, as described in U.S. Pat. Nos. 6,680,398, 7,151,188, 5,840,952 or US 2005/0124822, polysulphide species of this kind ought not to form or ought to form to a far lesser extent.
It is an object of the present invention to provide a process for preparing alkylpolyether-substituted mercaptosilanes that yields the alkylpolyether-substituted mercaptosilanes with weaker coloration.